1. Field of the Invention
The present invention relates to a toric multifocal lens and its related technique, which toric multifocal lens includes a plurality of vision correction regions that provide respective different values of astigmatism corrective optical power, the respective vision correction regions having different values of spherical optical power. In particular, the present invention is concerned with such a toric multifocal lens suitably used as an ophthalmic lens such as a contact lens or an intraocular lens, for correcting a lens wearer's vision which is deteriorated due to decreasing accommodation of the eye, and for correcting astigmatism. The present invention is also concerned with a method of producing the toric multifocal lens, and a cutting apparatus used for cutting a lens blank to form the toric multifocal lens.
2. Discussion of the Related Art
As disclosed in JP-A-57-105717, U.S. Pat. No. 4,636,049, JP-A-61-272717, U.S. Pat. No. 4,752,123, and U.S. Pat. No. 5,422,687, a multifocal lens is known as an ophthalmic lens used for an eye with decreasing accommodation due to presbyopia, or removal of a crystalline lens of the eye. The disclosed multifocal lens includes a plurality of vision correction regions whose centers are aligned with a common optical axis and which provide respective different values of spherical optical power. In addition to improve the decreasing accommodation of the eye, for the purpose of correcting astigmatism which results from distortion of the shape of the cornea or the crystalline lens, there is known a toric multifocal lens as disclosed in U.S. Pat. Nos. 4,580,882, 5,100,226, 5,805,260 and 5,847,802, wherein a plurality of vision correction regions having respective different values of spherical optical power include a toric portion for providing a suitable optical power for correction of astigmatism.
The optical power for correction of astigmatism, i.e., astigmatism corrective optical power, is determined based on a degree of astigmatism and an astigmatic axis of the eye of the patient. It is generally considered that an eye has a constant astigmatic degree and a single astigmatic axis. Accordingly, in the conventional toric multifocal lens, a cylindrical optical power and a cylindrical axis orientation which determine the astigmatism corrective optical power of the lens are made constant throughout the plurality of vision correction regions, corresponding to the astigmatic degree and astigmatic axis of the eye to be corrected. In particular, the cylindrical optical power and the cylindrical axis orientation of the conventional multifocal lens are usually determined on the basis of only the astigmatic degree and the astigmatic axis measured in a distant-viewing condition of the patient's eye.
The above-described conventional toric multifocal lens is, however, unsatisfactory to some patients whose eyes suffer from presbyopia as well as astigmatism, even when the conventional toric multifocal lens has a suitably determined spherical optical power for correction of presbyopia and a cylindrical optical power for correction of astigmatism. In particular, the conventional toric multifocal lens whose cylindrical optical power is determined on the basis of only the astigmatic data obtained in the distant-viewing condition does not assure a sufficiently high degree of visual acuity of the near objects, and the lens wearer undesirably feels an eye fatigue. Thus, the conventional toric multifocal lens is not capable of assuring sufficiently effective presbyopia and astigmatism correction performance with high stability.
As a result of a recent study in the field of the ophthalmic optics, it is found that the astigmatic condition in the eye (i.e., the astigmatic degree and the astigmatic axis) varies with various factors of the eye such as a change of the shape of the crystalline lens, which change change takes place due to a change in a distance from the wearer's eye to the desired object, namely, when the wearer's vision is shifted from the near objects to the distant objects, or vise versa, and due to a change of the diameter of the pupil, for instance. The inventors of the present invention made an extensive study based on the above findings, and found it effective to take into account the above-indicated changes in the astigmatic degree and axis, in determining the optical performance of the toric multifocal lens to be obtained for improving the accommodation of the eye decreased due to presbyopia, and for correcting astigmatism.